In order for a device in a wireless mobile communications system to be able to transmit signals that can be received by a base station in the network, it is often necessary for the device to transmit signals at specific times. The base station typically allocates specific time slots in which it can receive signals transmitted by devices, and so it is necessary for the devices to transmit those signals at times that take account of the time taken for the signal to propagate from the device to the base station.
Typically, there is a synchronization mechanism, by which a device can determine this propagation delay, and establish a value for a transmission time offset, so that it transmits its signals at times that ensure that the signals are received by the base station within the correct time window.
However, the synchronization mechanism takes a certain period of time, and requires messages to be exchanged between the device and the base station.
In some situations, it is advantageous for the device to be able to send messages with very low latency. For example, a sensor device may be configured so that it sends an alarm message when certain conditions are detected. Because the conditions are rare, the sensor device may not remain actively connected to the network. However, when the alarm conditions are detected, the alarm message should be sent with low latency and high reliability.